Embodiments of the invention relate generally to a switching device for switching off a current in a current path, and more particularly to micro-electromechanical system based switching devices.
To protect against damage, electrical equipment and wiring can be protected from conditions that result in current levels above their ratings. Over-current conditions can be classified by the time required before damage occurs and may be grouped into two categories: timed over-current conditions and instantaneous over-current conditions.
Timed over-current conditions or faults are deemed the less severe variety and generally require distribution protection equipment to deactivate the current path after a given time period, which depends on the level of the condition. Timed over-current faults typically include current levels just above the current rating, and may extend to and beyond 8-10 times the current rating of the distribution protection equipment. The system cabling and equipment can typically handle these conditions for a period of time, but the distribution protection equipment is designed to deactivate the current path if the current levels don't timely recede. Typically, timed faults can result from mechanically overloaded equipment or high impedance paths between opposite polarity lines (line to line, line to ground, or line to neutral).
Instantaneous over-current conditions, also termed short circuit faults, are severe faults and typically involve current levels greater than 10 times the rated current of the distribution protection equipment. These faults typically result from low impedance paths between opposite polarity lines. Short circuit faults involve extreme currents, can be extremely damaging to equipment and personnel, and therefore should be removed as quickly as possible. Minimizing response time, and thus the let-through energy, during a short circuit fault is of primary concern. Presently, two devices, fuses and circuit breakers, offer over-current protection for electrical equipment and wiring.
Fuses are typically more selective than circuit breakers and provide less variation in response to short circuit conditions, but must be replaced after they perform their protective functions. Fuses come in many shapes and sizes but are designed into fuse holders that allow them to snap-in and snap-out for ease of replacement. Manufacturers adhere to standard dimensions for the fuses and holders dependent on the fuse type and rating, making drop-in replacements easy.
Fuses are designed with series elements that melt at a prescribed overcurrent and thus open the current path. Fuses are thus by design single-phase devices, leading to potential issues when used in a poly-phase system, in which each fuse operates independent of the others. In many applications such as motor loads, losing one phase of power will lead to an increase in demand on the other phases. The increased demand on the other phases increases the risk of damage. For example motor loads may continue to run with a lost phase, causing additional heating and stress on the remaining phases.
For increased convenience, fuses have been replaced by circuit breakers in many applications. While circuit breakers provide similar protection and the convenience of being able to be reset rather than replaced after they operate or trip, they typically include complex mechanical systems with comparatively slow response times, in relation to fuses, and less selectivity between upstream and downstream circuit breakers during short circuit faults.
The electronic fault sensing method in breakers having electronic trip units typically involves some computation time that increases the decision time and thus reaction time to a fault. In addition, once the decision is made to trip, the mechanical systems are comparatively slow to respond due to mechanical intertia. Accordingly, in response to a short-circuit, a circuit breaker can allow comparatively larger amounts of energy (known as let-through energy) to pass through the circuit breaker.
A contactor is an electrical device designed to switch an electrical load ON and OFF on command. Traditionally, electromechanical contactors are employed in control gear, where the electromechanical contactors are capable of handling switching currents up to their interrupting capacity. Electromechanical contactors may also find application in power systems for switching currents. However, fault currents in power systems are typically greater than the interrupting capacity of the electromechanical contactors. Accordingly, to employ electromechanical contactors in power system applications, it may be desirable to protect the contactor from damage by backing it up with a series device that is sufficiently fast acting to interrupt fault currents prior to the contactor opening at all values of current above the interrupting capacity of the contactor.
Previously conceived solutions to facilitate use of contactors in power systems include vacuum contactors, vacuum interrupters and air break contactors, for example. Unfortunately, contactors such as vacuum contactors do not lend themselves to easy visual inspection as the contactor tips are encapsulated in a sealed, evacuated enclosure. Further, while the vacuum contactors are well suited for handling the switching of large motors, transformers and capacitors, they are known to cause undesirable transient overvoltages, particularly when the load is switched off.
Furthermore, the electromechanical contactors generally use mechanical switches. However, as these mechanical switches tend to switch at a relatively slow speed, predictive techniques are employed in order to estimate occurrence of a zero crossing, often tens of milliseconds before the switching event is to occur, in order to facilitate opening/closing at the zero crossing for reduced arcing. Such zero crossing prediction is prone to error as many transients may occur in this prediction time interval.
As an alternative to slow mechanical and electromechanical switches, fast solid-state switches have been employed in high speed switching applications. As will be appreciated, these solid-state switches switch between a conducting state and a non-conducting state through controlled application of a voltage or bias. For example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. However, since solid-state switches do not create a physical gap between contacts when they are switched into a non-conducing state, they experience leakage current. Furthermore, due to internal resistances, when solid-state switches operate in a conducting state, they experience a voltage drop. Both the voltage drop and leakage current contribute to the generation of excess heat under normal operating circumstances, which may effect switch performance and life. Moreover, due at least in part to the inherent leakage current associated with solid-state switches, their use in circuit breaker applications is not practical.
Accordingly, there exists a need in the art for a current switching circuit protection arrangement to overcome these drawbacks.